It is common in various areas of manufacture, such as in the case of punches; and hotplate, ultrasonic or vibration welding, staking, inserting and the like, to provide a head that is caused by a press to move, for example up and down, with respect to a workpiece upon which work is to be performed. It is also common for the press, via the head and typically a tool, to exert a great amount of force upon the workpiece, and for it to be necessary for a human operator to be required to manually adjust the position of the workpiece with respect to the head as the head approaches the workpiece in order to achieve proper working alignment.
More specifically, ultrasonic welding is a process whereby ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. During this process the work pieces to be welded together are held under pressure between a fixed anvil and a horn which emits the acoustic vibrations. The pressure generated between the anvil and the horn during the welding process can be quite great. A problem exists, however, in that it is often necessary for an operator to be required to manually position the parts for welding, and if the operator's hand, or more typically one or more of his/her fingers, is caught between the head or anvil and the work piece, the large force exerted by the press can cause severe and permanent damage to the operator.
A number of safety devices, such as guards, sensors and the like, have been developed to reduce the occurrence of operator injury. However, all known safety devices have problems of their own, such as interfering with the operator's manipulation of the workpiece, requiring expensive add-ons to the press and being subject to tampering and/or removal.
One such safety device is disclosed in U.S. Pat. No. 6,418,824, which is a hydraulic press that utilizes a single actuator and is powered by a first low pressure source, to move the press safely to a predetermined position without generating enough force to seriously injure or damage any obstructions. Once the piston has reached its predetermined position, a second high pressure source is supplied to the actuator to complete the cycle. However, this system requires the machine to have two separate pressure sources, which adds a great deal of cost and inefficiency to the process. Furthermore, there are risks of these pressure sources being improperly adjusted or possibly switched, which could result in harm to the operator.
What is desired, therefore, is a press which provides the safety advantage of a low force and high force mode but only uses a single fixed pressure source.